The invention is directed to an electrolytic bath for the deposition of haze-free glossy rhodium coatings which consist essentially of rhodium sulfate or phosphate, sulfuric acid, and/or phosphoric acid.
For decorative rhodinizations, especially for overlaying white gold there are needed rhodium coatings which have a haze-free high gloss and an especially bright gray tint which is similar to the white-gold color. The film thickness of this coating is between 0.1 and 1.mu..
For the deposition of electrolytic rhodium films there are chiefly used baths which contain phosphate or sulfate and sulfuric acid or phosphoric acid. If films having a thickness of more than 0.7 .mu.m are deposited then it is not possible to deposit the required haze-free glossy coatings without further additives. Also it is only possibly with difficulty to reproducibly obtain the light gray tint of the film with the known baths, even if the necessary conditions in the production of the rhodium preparation are exactly observed.
Besides coatings from these bath exhibit high internal stress so that even at very thin films numerous cracks occur in the film. The corrosion protecting action of the rhodium film is greatly reduced thereby.
There are already known rhodium baths having various additives which are supposed to improve the mentioned disadvantageous properties of the deposited rhodium films. Metallic additives such as thallium (Swiss Pat. No. 553255) or copper (French Pat. No. 1577593) have the disadvantage that they are hight toxic or do not give the desired bright tints.
In the use of alkali metal chlorides, as well as MgCl.sub.2 or AlCl.sub.3 (German OS No. 2329578) some chlorine can form at the anode through which the use is made substantially more difficult. Besides it is difficult to produce constant deposition conditions through the continuous discharge of chloride.
The use of organic compounds in rhodium baths as a rule leads to films which to be sure can be glossy but which no longer exhibit the bright color.
Thus there is known a rhodium bath (German Pat. No. 627,264) which can contain benzoic acid, phenol solution and gelatin. Furthermore polybasic organic acids, e.g. glutaric acid are known as additives (German OS No. 2242503). However, rhodium films produced from these baths do not show the desired white gold color.
Therefore it was the problem of the present invention to find an electrolytic bath for the deposition of haze-free glossy rhodium coatings which consists essentially of rhodium sulfate or phosphate, sulfuric acid, and/or phosphoric acid and furnishes a white gold-like tint and films with low intonal stress.